It is proposed to establish whether masking of histocompatibility antigens by large glycoprotein molecules at the cell surface is a mechanism by which a tumor may avoid destruction by the host. To test the validity of this mechanism, previously suggested for the nonstrain- specific mammary carcinoma, TA3-Ha, of the strain A mouse, a series of 12 hybrid cell lines of varied immunological characteristics, each cloned from hybrid cells which resulted from fusion of TA3-Ha tumor cells with normal embryonic fibroblasts of the ACA mouse, will be quantitatively studied for a correlation between: (1) absorption of antisera to histocompatibility antigens (H-2a) and (2) the presence at the cell serface of the large rod-like TA3-Ha glycoprotein, epiglycanin. The total amount and composition of epiglycanin and surface sialic acid, as well as antisera absorption, will be determined for each hybrid cell line.